1. Field of the Invention
The present invention relates to an apparatus for sensing the angular displacement, or extent of partial rotation, of a member and more particularly to such a sensing apparatus which utilizes electrical inductance as the technique for sensing angular displacment.
2. Description of the Prior Art
In various applications it is desirable to monitor the angular displacement of a first object which is rotatably positioned relative to a second object. For example, it is desirable to sense the angular displacement of various components of road grading or paving machines so that the completed roadway has the desired height above a reference such as a string line or existing pavement. In other machinery, as well, it is necessary or desirable to monitor whether one component has moved angularly relative to a second component in order, for instance, to provide a control function to maintain the components level or at a desired angular displacement relative to one another.
The determination of angular displacement is relatively more sensitive and complex than the monitoring of revolutions of a rotating shaft or the like, which is normally accomplished by a tachometer. Furthermore, in the abovementioned applications, it is necessary that the angular displacement sensor be rugged and reliable, yet capable of sensing relatively fine changes in relative angular positioning. Further, because positional adjustments may be based upon the sensed angular displacement, it is most desirable that the output of the sensor be linearly related to the angular position; such linearity ordinarily substantially simplifies any adjustments which must be made to restore, for example, a level relationship.
In the prior art, it is known to sense angular displacements with potentiometric sensors, inductive displacement transducers (such as rotary variable differential transformers), and induction potentiometers. The prior art sensors based upon potentiometric measurements, while accurate, frequently prove to be unreliable because of the unavoidable frictional engagement between a wiper element and a resistive element. Likewise, conventional inductive sensors are usually expensive and difficult to calibrate; for example, inductive displacement transducers utilizing rotary variable differential transformers employ complex rotating parts and a multiplicity of coils which can cause complications in calibration.